draganddrop replace QList::first() with constFirst()
Fix clazy issues "src/qmlcontrols/draganddrop/DeclarativeMimeData.cpp:61:16: warning: Don't call QList::first() on temporary [clazy-detaching-temporary] src/qmlcontrols/draganddrop/MimeDataWrapper.cpp:30:16: warning: Don't call QList::first() on temporary [clazy-detaching-temporary]".